The keep function in Clojure invokes a function on each item in a collection and only returns non-nil results from the function invocation. The result of the keep function is a lazy sequence.

In Clojure we can use the rand-nth function to get a single random element from a sequence. To get multiple items based on random probability for each item we use the function random-sample. We must set the probability that determines for each item if it is in the result or not.

We can use the flatten function when we have a collection with nested sequential collections as elements and create a new sequence with the elements from all nested collections.

In the clojure.set namespace we can find the intersection function. This functions accepts one or more sets as arguments and return a new set with all elements that are present in the sets that are passed as arguments to the intersection function. The argument must be a set, so we need to convert other collection or seq values to a set first before we use it as an argument for the function.

We can use the join function from the clojure.string namespace to join elements from a collection into a string. We can optionally specify a separator that is used to separate each element in the string output. The separator is not used after the last element of the collection. If we don’t specify a separator the elements are concatenated without separation. The string representation for each element in the collection is used in the joined end result.